Method for through-plating field effect transistors with a self-assembled monolayer of an organic compound as gate dielectric

ABSTRACT

A method for through-plating field effect transistors having a self-assembled monolayer of an organic compound as gate dielectric includes through-plated by patterning a gate electrode material, and bringing an organic compound having dielectric properties into contact with the contact hole material and the gate electrode material. A contact hole material and the gate electrode material are at least partially uncovered. The contact hole is material not identical to the gate electrode material. A self-assembled monolayer of the organic compound is formed above the gate electrode material. The method also includes depositing and patterning the source and drain contacts without removing the self-assembled monolayer of the organic compound, and depositing a semiconductor material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119 to German ApplicationNo. DE 10 2004 008 784.9, filed on Feb. 23, 2004, and titled “Method forthe Through-Plating of Field Effect Transistors with a Self-AssembledMonolayer of an Organic Compound as Gate Dielectric,” the entirecontents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to organic field effect-transistors, andmore particularly, to a method for through-plating field effecttransistors.

BACKGROUND

Field effect transistors based on organic semiconductors are of interestfor electronic applications that require extremely low production costs,flexible, or unbreakable substrates, or fabrication of transistors andintegrated circuits over large areas. For example, organic fieldtransistors are used as pixel control elements in active matrix screens.Such screens are usually produced with field effect transistors based onamorphous or polycrystalline silicon layers. Temperatures of more than250° C. usually necessary for fabricating high-quality transistors basedon amorphous or polycrystalline silicon layers require the use of rigidunbreakable glass or quartz substrates. Due to the relatively lowtemperatures at which transistors based on organic semiconductors arefabricated, e.g., usually less than 200° C., organic transistors permitproduction of active matrix screens using inexpensive, flexible,transparent, unbreakable polymer films with considerable advantages overglass or quartz substrates.

Another application for organic field effect transistors is fabricationof very inexpensive integrated circuits used, for example, for activelabeling and identification of merchandise and goods. These transpondersare usually produced using integrated circuits based on monocrystallinesilicon, which leads to considerable costs in the construction andconnection technology. Producing transponders based on organictransistors leads to enormous cost reductions and assists in increasingtransponder technology en route to a worldwide breakthrough.

The fabrication of thin-film transistors usually requires a large numberof steps in which the different layers of the transistor are deposited.In a first step, the gate electrode is deposited on a substrate, thenthe gate dielectric is deposited on the gate electrode, and the sourceand drain contacts are deposited and then patterned. Finally, thesemiconductor is deposited between the source and drain electrodes onthe gate dielectric.

Efforts to simplify the fabrication process for field effect transistorsand to fabricate the field effect transistors with smaller dimensionshave been made. Fabrication of organic field effect transistors requiresa targeted patterning of the gate electric layer since the targetedaccess necessary for operation of the transistors to the electrodes orcontacts in the metallization plane or the metallization planes belowthe insulating layer are produced by through-plating (also referred toas contact hole or “via”) in the insulating layer. An access to themetallization planes situated below the insulating layer is necessary,if the input of one transistor is to be linked to the output of anothertransistor, as is necessary in many cases in almost every integratedcircuit.

In recent years, a plurality of microelectronics elements have beendescribed which have a size of a few nanometers and require nolithographic methods or fewer lithographic steps to be fabricated. Theseelements are nanoelements fabricated by nanotechnology. The elementshave a self-assembled molecular layer (self-assembled monolayer).

When using conventional gate dielectrics, such as silicon oxide andaluminum oxide, for example, the contact holes are opened afterdeposition of the initially closed dielectric layer. For patterning thegate dielectric, a photoresist is applied, exposed, and developed. Then,the gate dielectric is removed by an etching process in regions of theenvisaged contact holes. The photoresist protects the regions that arenot to be etched. Finally, the photoresist mask is again removed.

This method is unsuitable for patterning molecular self-assembledmonolayers. Photoresists are generally developed in a basic solution,which leads to destruction of the monolayer. Coating monolayers withresist is difficult due to strongly hydrophobic nature of manyself-assembled monolayers.

SUMMARY

A method for through-plating field effect transistors having aself-assembled monolayer arranged on the gate electrode and serving as agate dielectric includes patterning a gate electrode material to formcontact hole for thorough-plating the gate electrode depositing of anorganic compound having dielectric properties above the contact holematerial and the gate electrode material, depositing and patterning thesource and drain contacts without removing the self-assembled monolayer,and depositing a semiconductor material. A contact hole material suchthat the contact hole material and the gate electrode material are atleast partly uncovered. The contact hole material is not identical tothe gate electrode material. A self-assembled monolayer of the organiccompound is formed selectively above the gate electrode material. Thethrough plating of field effect transistors is simplified and compatiblewith conventional lithographic techniques.

In the method according to the invention, the self-assembled monolayerserving as a gate dielectric is processed such that providingplated-through holes is effected as early as during deposition of themonolayer and renders subsequent patterning of the monolayersuperfluous, by using the selectivity of the absorption of molecularmonolayers in dependence on configuration of the substrate surface. Themolecular monolayer is formed only above the gate electrode material andnot above the contact hole material. This is possible because manymetals have a native oxide layer that can be used for the selectivity.No absorption of the organic compounds then occurs on noble metals sincenoble metals do not have a metal oxide layer necessary for formation ofthe self-assembled monolayer. Through the targeted selection of the gateelectrode material, the contact hole material and the organic compoundmaterials are selected so that self-assembled monolayer is formed onlyabove the gate electrode material.

If, for example, the gate electrode material is selected from the groupconsisting of aluminum (Al), titanium (Ti), titanium nitride (TiN),tantalum (Ta), tantalum nitride (TaN), tungsten (W), titanium tungsten(TiW), tantalum tungsten (TaW), tungsten nitride (WN), tungstencarbonitride (WCN), iridium oxide (IrO), ruthenium oxide (RuO), andstrontium ruthenium oxide (SRuO), the contact material may be selectedfrom the group of the noble metals, such as, for example, gold (Au),platinum (Pt), palladium (Pd), silver (Ag), and from gallium arsenideand indium phosphide. In this case, a self-assembled monolayer of anorganic compound selected from the group of the phosphonic acidderivatives may be used as the gate dielectric since the phosphonic acidderivatives adsorb in a targeted manner on the surface of the base,natively oxidized metals and not on the surface of the noble metals.Since the surfaces of the gate electrode material and of the contacthole material are partly uncovered, during the self-assembly of theorganic compound, a self-assembled monolayer is formed on the gateelectrode material, whereas no self-assembled monolayer is formed abovethe contact hole material.

In an exemplary implementation of the invention,;the gate electrodematerial and the contact hole material are selected from the materialsmentioned above.

However, it is also possible for the contact hole material to beselected from the group consisting of aluminum (Al), titanium (Ti),titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), tungsten(W), titanium tungsten (TiW), tantalum tungsten (TaW), tungsten nitride(WN), tungsten carbonitride (WCN), iridium oxide (IrO), ruthenium oxide(RuO), and strontium ruthenium oxide (SRuO). In this case, the materialof the gate electrode is selected from the group consisting of noblemetals, such as, for example, gold (Au), platinum (Pt), palladium (Pd),and silver (Ag), and from gallium arsenide and indium phosphid. In thiscase, the self-assembled monolayer of the organic compound is intendedto have groups that form a self-assembled monolayer in a targeted manneron the surface of the noble metals and not on the surface of the basemetals. Such an organic compound may, for example, have radicalsselected from the group consisting of SH, OH, NH₂, NHR, NR₂, COOH,CONH₂, CN, CONHOH, CONHNH₂ or PR₂. In an embodiment of the invention,the gate electrode material is selected from the group of noble metalsand from gallium arsenide and indium phosphide.

Even though metal oxide is mentioned as a preferred embodiment as thepossibility for obtaining selectivity, other groups having a selectivitywith respect to an organic compound may also be introduced through atargeted treatment of the gate electrode material. Such a treatment ispossible, for example, if the gate electrode material includes silicon.Silicon can then be treated in such a way that different groups thatinteract with an organic compound are present at the surface, so thatthe self-assembled monolayer is formed only above the gate electrodematerial. Silicon may then have groups, such as, for example, —H, —OH,or —NH₂.

In a particular embodiment, however, a metal oxide layer is preferredsince this layer is already present or is relatively easy to fabricatein the case of many non-noble metals.

However, if the gate electrode material is selected from the group ofthe noble metals as described above, the contact hole material has ametal oxide layer, so that an organic compound having SH groups, forexample, is formed only on the surface of the noble metal.

The organic compound that forms a self-assembled monolayer above thegate electrode material can be selected from a multiplicity ofcompounds. Selectivity between the gate electrode material and thecontact hole material is a precondition. For example, the organiccompound may have a radical selected from the group consisting ofR—SiCl₃, R—SiCl₂alkyl, R—SiCl(alkyl)₂, R—Si(OR)₃, R—Si(OR)₂alkyl,R—SiOR(alkyl)₂, R—PO(OH)₂, R—CHO, R—CH═CH₂, R—SH, R—OH, R—NH₂, R—COOH,R—CONH₂, R—CONHOH, R—CONHNH₂, and R—CN, where R may be an arbitrarygroup, and in particular, an n-alkyl, n-alkyl ether, a linear aromaticgroup of the formula —(C₆H₄)n—, where the alkyl and alkyl ether groupshave between 4 and 40 C atoms and n is an integer between 2 and 6.

Since, as gate electrode material, metals which have either a nativemetal oxide layer or a metal oxide layer that can easily be fabricatedare conventionally selected from the group, such as aluminum ortitanium, for example, in a preferred embodiment the organic compoundhas a group selected from R—PO(OM)₂, where R has the above meaning. Mmay be either H, a metal or an arbitrary organic group.

The material for the source and drain contacts may be an arbitrarymaterial and the selection is not critical for the present invention.

The semiconductor material may be both of inorganic and of organicnature. In a preferred embodiment, however, the semiconductor materialis an organic polymer.

In the preferred embodiment, the organic polymer is selected from thegroup consisting of pentacene, tetracene and polythiophene.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to thefigures.

FIGS. 1A-1E show the process sequence for fabricating an organic fieldeffect transistor with a molecular self-assembled monolayer as gatedielectric;

FIGS. 2A and 2B show a schematic cross section of an organic fieldeffect transistor and a contact hole, fabricated in accordance with themethod according to the invention.

FIGS. 3A and 3B show current-voltage characteristic curves of a fieldeffect transistor according to the invention.

DETAILED DESCRIPTION

FIGS. 1A-1E show a cross section of a gate electrode that can befabricated from the gate electrode material according to the invention.The structure illustrated in FIG. 1B is obtained after patterning thecontact hole and depositing the contact hole material. The gateelectrode material and the contact hole material simultaneously contactan organic compound. A self-assembled monolayer of the organic compoundis formed selectively above the gate electrode material (and not abovethe contact hole material) (FIG. 1C). The self-assembled monolayer ofthe organic compound serves as gate dielectric, so that the source anddrain contacts are deposited and patterned without using a photomask forpatterning the gate dielectric. The result is illustrated in FIG. 1D.Finally, a semiconductor is deposited in order to attain the structureillustrated in FIG. 1E.

It is possible to first define the metal for the gate electrode and thento define the metal for the contact holes, or, conversely, to firstdefine the metal for the contact holes and then define the metal for thegate electrodes. The contact hole material needs to be at leastpartially uncovered after defining of the gate electrode material.

FIGS. 2A and 2B illustrate schematic cross-sections of an organic fieldeffect transistor (left) and of a contact hole (right), which arefabricated using the method according to the invention. In thearrangement shown in FIG. 2A, the metal for the gate electrodes wasfirst defined and then the metal for the contact holes was defined. Inthe arrangement shown in FIG. 2B, the metal for the contact holes wasdefined first and then the metal for the gate electrodes was defined.

EXAMPLE

A layer of aluminum having a thickness of 20 nm is vapor-deposited ontoa glass substrate and patterned by photolithography and wet-chemicaletching in a weakly basic solution in order to define the gateelectrodes. A layer of gold having a thickness of 20 nm is thenvapor-deposited and patterned by photolithography and wet-chemicaletching in an iodine potassium iodide solution in order to define thecontact holes. Afterward, the substrate is dipped into an alcoholicsolution of the phosphonic acid derivative n-octadecyl phosphonic acid(C₁₈H₃₇PO(OH)₂) in order to produce the gate dielectric, a molecularmonolayer is formed on the aluminum gate electrodes, not in the contactholes. A layer of gold with a thickness of 20 nm is subsequentlydeposited and patterned in order to produce the source and draincontacts. Finally, pentacene is vapor-deposited.

FIGS. 3A and 3B show the current-voltage characteristic curves of apentacene transistor and the output signal of a five-stage pentacenering oscillator which, as described above, are fabricated by the methodsaccording to the invention.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. Accordingly, it is intendedthat the present invention covers the modifications and variations ofthis invention provided they come within the scope of the appendedclaims and their equivalents.

1. A method for through-plating field effect transistors, comprising:patterning a gate electrode material to form a contact hole for thethrough-plating of a gate electrode, a contact hole material and thegate electrode material being at least partially uncovered, the contacthole material not being identical to the gate electrode material; anorganic compound having dielectric properties contacting the contacthole material and the gate electrode material, a self-assembledmonolayer of the organic compound being formed above the gate electrodematerial; depositing and patterning the source and drain contactswithout removing the self-assembled monolayer of the organic compound;and depositing a semiconductor material.
 2. The method as claimed inclaim 1, wherein if the material for the gate electrode is selected fromthe group consisting of Al, Ti, TiN, Ta, TaN, W, TiW, TaW, WN, WCN, IrO,RuO, and SrRuO, the contact hole material is selected from the groupconsisting of gallium arsenide, indium phosphide, Au, Pt, Pd, and Ag. 3.The method as claimed in claim 1, wherein if the contact hole materialis selected from the group consisting of Al, Ti, TiN, Ta, TaN, W, TiW,TaW, WN, WCN, IrO, RuO, and SrRuO, the gate electrode material isselected from the group consisting of gallium arsenide, indiumphosphide, Au, Pt, Pd, and Ag.
 4. The method as claimed in claim 1,wherein the gate electrode material has a metal oxide layer at thesurface.
 5. The method as claimed in claim 1, wherein the contact holematerial has a metal oxide layer at the surface.
 6. The method asclaimed in claim 2, wherein the organic compound has a radical selectedfrom the group consisting of R—SiCl₃, R—SiCl₂alkyl, R—SiCl(alkyl)₂,R—Si(OR)₃, R—SI(OR)₂alkyl, R—SiOR(alkyl)₂, R—PO(OH)₂, R—CHO, R—CH═CH₂,SH, OH, NH₂, COOH, CONH₂, CONHOH, CONHNH₂, and CN, where R is one of anarbitrary group, an n-alkyl, n-alkyl ether, or a linear aromatic groupof the formula —(C₆H₄)n—, where the n-alkyl and n-alkyl (thio)ethergroups have between 4 and 20 C atoms and n is an integer between 2 and6.
 7. The method as claimed in claim 2, wherein the organic compoundcorresponds to the formula R—PO(OM)₂, where R is one of an arbitrarygroup, an n-alkyl, n-alkyl ether, or a linear aromatic group of theformula —(C₆H₄)n—, where the n-alkyl and n-alkyl (thio)ether groups havebetween 4 and 20 C atoms and n is an integer between 2 and 6 and M isone of H, metal, or an organic radical.
 8. The method as claimed inclaim 1, wherein the semiconductor material is an organic polymer. 9.The method as claimed in claim 8, wherein the organic polymer isselected from the group consisting of pentacene, tetracene, andpolythiophene.
 10. The method as claimed in claim 2, wherein the gateelectrode material has a metal oxide layer at the surface.
 11. Themethod as claimed in claim 3, wherein the contact hole material has ametal oxide layer at the surface.
 12. The method as claimed in claim 4,wherein the organic compound has a radical selected from the groupconsisting of R—SiCl₃, R—SiCl₂alkyl, R—SiCl(alkyl)₂, R—Si(OR)₃,R—SI(OR)₂alkyl, R—SiOR(alkyl)₂, R—PO(OH)₂, R—CHO, R—CH═CH₂, SH, OH, NH₂,COOH, CONH₂, CONHOH, CONHNH₂, and CN, where R is one of an arbitrarygroup, an n-alkyl, n-alkyl ether, or a linear aromatic group of theformula —(C₆H₄)n—, where the n-alkyl and n-alkyl (thio)ether groups havebetween 4 and 20 C atoms and n is an integer between 2 and
 6. 13. Themethod as claimed in claim 3, wherein the organic compound has a radicalselected from the group consisting of R—SiCl₃, R—SiCl₂alkyl,R—SiCl(alkyl)₂, R—Si(OR)₃, R—SI(OR)₂alkyl, R—SiOR(alkyl)₂, R—PO(OH)₂,R—CHO, R—CH═CH₂, SH, OH, NH₂, COOH, CONH₂, CONHOH, CONHNH₂, and CN,where R is one of an arbitrary group, an n-alkyl, n-alkyl ether, or alinear aromatic group of the formula —(C₆H₄)n—, where the n-alkyl andn-alkyl (thio)ether groups have between 4 and 20 C atoms and n is aninteger between 2 and
 6. 14. The method as claimed in claim 5, whereinthe organic compound has a radical selected from the group consisting ofR—SiCl₃, R—SiCl₂alkyl, R—SiCl(alkyl)₂, R—Si(OR)₃, R—SI(OR)₂alkyl,R—SiOR(alkyl)₂, R—PO(OH)₂, R—CHO, R—CH═CH₂, SH, OH, NH₂, COOH, CONH₂,CONHOH, CONHNH₂, and CN, where R is one of an arbitrary group, ann-alkyl, n-alkyl ether, or a linear aromatic group of the formula—(C₆H₄)n—, where the n-alkyl and n-alkyl (thio)ether groups have between4 and 20 C atoms and n is an integer between 2 and
 6. 15. The method asclaimed in claim 4, wherein the organic compound corresponds to theformula R—PO(OM)₂, where R is one of an arbitrary group, an n-alkyl,n-alkyl ether, or a linear aromatic group of the formula —(C₆H₄)n—,where the n-alkyl and n-alkyl (thio)ether groups have between 4 and 20 Catoms and n is an integer between 2 and 6 and M is one of H, metal, oran organic radical.